Triway Crossovers

In theory, the speakers connected to each channel can utilize the stereo output of the amplifier and the speaker bridged across the two channels can utilize the bridged capability.  If it's a 100 watt per channel into 4-ohm amplifier, each 4-ohm speaker connected in stereo can utilize up to 100 watts and a bridged 8-ohm speaker could utilize up to 200 watts.  Or whatever the amplifier is rated for.  If the bridged speaker is 4-ohms, it could draw as much as 400 watts from the amplifier (in theory.)

In practice, however, this is not strictly true for a couple of basic reasons.  First the passive crossover will absorb a small percentage of the power output, and second due to the frequency distribution, about 70% of amplifier power is going to bass frequencies so depending on the crossover frequency, the main stereo set may actually only use about 30 watts per channel.  Plus operating in tri-mode is very hard on most amps and you will suffer from a high likelihood of clipping, power compression, heat losses, etc.  But with a proper crossover and careful gain setup, it can sound OK for a low-budget solution.